1. Field of the Invention
The present invention relates to a semiconductor storage device, and more particularly, to a semiconductor storage device including a power-on reset circuit inside thereof.
2. Description of Related Art
In a system including a semiconductor storage device (hereinafter referred to as memory), the whole system may be hung up by unexpected events (system abnormal operation, power supply drop due to noise, and so on) during the operation. In this case, it is needed to execute a soft reset by external control as a memory setting state becomes unclear, so as to reliably reset the internal part of the memory.
FIG. 3 is a diagram showing a configuration example of a semiconductor device of a memory according to a related art. When the power is turned on, an initialization process of each circuit is executed by a power-on reset signal (PONZ signal). Further, when a mode register set command is input from outside after the power is turned on, the soft reset resetting each circuit other than an internal power supply generator 20p is executed by a mode register set signal (MRSPON signal) output from an MRS controller 64p. 
For example, Japanese Unexamined Patent Application Publication No. 2004-103222 discloses a technique of resetting a block related to a data path employing an external signal applied at a certain level in the soft reset. According to this technique, data conflicts or ineffective data can be prevented in executing operations according to the read/write commands applied after the soft reset.
Japanese Unexamined Patent Application Publication No. 01-137494 discloses a technique of automatically generating a reset signal to initialize a memory control part when a circuit of the memory control part falls into an unoperable state.
Further, Japanese Unexamined Patent Application Publication No. 2006-252654 discloses a technique of reliably resetting an internal circuit of a semiconductor memory by external control. In this technique, a mode register set by a predetermined bit is executed by employing a technique of applying the mode register setting the operation mode of the memory, in order to generate the soft reset signal and reset the internal circuit. As such, it is possible to reliably reset the internal circuit of a memory.
However, according to the reset technique in which the signal same to the power-on signal generated in the starting operation is again generated from the mode register set command, the power-on signal from the mode register set command resets the system of the whole memory. Since the power-on signal typically needs to be generated only in the starting operation, the power-on signal is generated when voltage lower than the voltage supplied to the device of normal operation specifications is detected. Thus, when the power-on signal is again generated from the mode register set command, the soft reset cannot be accurately operated. This is because the power supply has already been supplied enough and the system of the whole memory is reset at a voltage higher than the starting operation; therefore, the power consumption increases and the reset operation itself becomes a source of the noise.
Further, in recent years, the memory capacity has been increasing and the circuit configuration has been complicated, which increases the time required for the starting operation after the power-on. When the system of the whole memory is reset, the time required for the recovery increases as well. Accordingly, a function of resetting the controller setting the internal operation from the mode register set command has been demanded.